Because radiographic imaging, including computed tomography (CT) has the capability of detecting small low contrast features, it has become an integral part of radiology medical practice, allowing medical practitioners to detect low contrast tumors and lesions in soft tissue anatomical regions including the brain and the liver. Low contrast detection is an important characteristic of diagnostic x-ray imaging methods. An important issue in radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. Generally, higher radiation doses result in the ability to detect lower contrast smaller objects, while lower doses lead to increased image noise. Increased radiation dosage increases the risk of radiation induced cancer.
The descriptions in this patent are focused on computed tomography (CT) but the application of the technology described is not limited to CT. The technology described can be applied to other radiographic imaging systems as well so that references in this patent to “CT” or “CT scanner” should be interpreted as applying to other radiographic imaging systems.
The ability of a CT system to differentiate a low-contrast object from its background is measured by its low contrast detectability (LCD). LCD is measured with phantoms that contain low-contrast objects of various sizes. Phantoms that produce low contrast objects by using materials with different densities are useful for testing conventional energy integrating CT scanners. Phantoms that produce low contrast objects using energy sensitive materials would allow performance testing for a dual energy scanner.
The low-contrast resolution of a CT scanner is generally defined as the smallest object that can be detected at a given contrast level and dose. The contrast level is usually specified as a percentage of the linear attenuation coefficient of water. A sample specification with the current method might be “4 mm at 0.3% contrast for 10 mm slice thickness at 30 mGy CTDIvol dose.” Sometimes other dose metrics are used such as the surface dose measured at the outer surface of the phantom.
The current LCD specification is made at a single protocol in one of two ways:    1. human observation—reconstructed images are viewed by one or more human observers to determine the smallest pin that is visible in the opinion of the observer;    2. statistical method—an automated algorithm predicts from a flat “water” image the contrast required to detect a given size pin with a specified confidence interval.
In this description, it will be shown that the current LCD specification is inadequate in that it characterizes the performance of the CT scanner at only one protocol and that it is necessary to characterize the performance over an extended range including, for example, the full operating range of the scanner.
Contrast Index
In order to extend the measurement of low contrast detectability, a new contrast measure, M can be used, as defined in one way for example by Equation 1,
                              M          =                                    M              0                        cp                          ,                            (        1        )            and designated as “contrast index.” In Equation 1, p is the smallest pin size, measured in millimeters, visible at contrast level, c, measured in Hounsfield units (HU) where one Hounsfield unit corresponds to 0.1% of water attenuation and M0 is an arbitrary constant for bringing the measure, M, into a convenient numerical range. It is important to note that the contrast level, c, in this definition is the nominal or expected contrast level of the object as opposed to a measured contrast level, later indicated with an upper case C. In this example, M0=6000 in order to map the best current contrast specification of 2 mm at 0.3% to a contrast measure of 1000. For example, the specification, “4 mm at 0.3% contrast for 10 mm slice thickness at 30 mGyCTDIvol,” would generate a contrast measure of 500,
                    M        =                              6000                                          (                3                )                            ⁢                              (                4                )                                              =          500.                                    (        2        )            Flux Index
Commercial CT scanners typically operate over a wide range of protocols, each of which can have distinct contrast characteristics. The protocol parameters that impact contrast include (1) scan time, (2) tube current (mA), (3) slice thickness, (4) object diameter, (5) tube voltage (kVp) and (6) x-ray filter. Also, contrast is significantly impacted by non-linear reconstruction methods as well as the reconstruction pixel size and reconstruction filter. It is assumed in the following that the tube voltage, the x-ray filter, the scan diameter and the reconstruction method, collectively comprising a core operating mode, are fixed and that the scanner, in that core operating mode, can be characterized by the CTDIvol dose index. Then the parameters that directly affect the x-ray flux available for detection are:
1. scan time (0.25-2.0 sec/revolution)
2. x-ray tube current (20-400 mA)
3. slice thickness (0.5-10.0 mm)
4. object diameter (20-50 cm)
5. dose index (CTDIvol)
A relative flux measure, designated as the “flux index,” incorporates these 5 parameters as follows.
                              FluxIndex          =                                    CTDIvol                              CTDIvol                ref                                      *            mA            *            sliceThick            *            scanTime            *                                          ⅇ                                                      -                    objDiam                                    *                  attWater                                                            ⅇ                                                      -                    refDiam                                    *                  attWater                                                                    ⁢                                  ⁢                                  ⁢                  refDiam          =                      20.0            ⁢                                                  ⁢            cm                                              (        3        )            CTDIvol is per 100 mAs and CTDIvolref is an arbitrary constant dose reference value per 100 mAs that will be determined for each core operating mode tested. For practical combinations of these parameters, the range of Flux Index is approximately [0.1, 7,000.0]. An example of a current LCD specification could be “4 mm at 0.3% for 10 mm slice at 90 mAs.” Since this example relates to the 20 cm CATphan, the Flux Index would be 900.
The relative flux index, described above, relates linearly to dose except for the factor involving the object diameter. The currently accepted dose index for CT is CTDIvol as defined in IEC 60601-2-44. Dose is linearly related to flux for a given object size and slice thickness. The contrast measurements discussed in this paper are generally accomplished at the center of the object. For that reason, the derivation and the description of the ExLCD method is currently based on the relative flux index.
ExLCD Graph
As described above, the range of flux index for a CT scanner is approximately [0.1, 7,000.0]. It can be demonstrated that the corresponding range of contrast index is approximately [0.5, 1000.0]. These ranges define the scope of the ExLCD graph, shown in FIG. 27 in log-log format.
Larger values of Contrast Index indicate better image quality or the ability to detect smaller, lower contrast objects. Smaller values of Contrast Index indicate poorer image quality or the ability to detect only larger, higher contrast objects.
Larger values of Flux Index indicate higher dose or smaller patient sizes. Smaller values of Flux Index indicate lower dose or larger patient sizes.
The current LCD methods often utilize the CTP515 low contrast module (FIG. 5) of the CATphan phantom (FIG. 4). [The Phantom Laboratory, http://www.phantomlab.com/pdf/catphan600_download.pdf] The “supra-slice” contrast sets are used but only the lowest 0.3% contrast set is typically reported.
There are two LCD measurement methods currently used on commercial CT scanners: (1) human observer method and (2) statistical method. We have compiled some recent reported measurements from the major CT manufacturers and collected them in Table 1. [NHS Purchasing and Supply Agency, Buyer's Guide, Computed Tomography Scanners, Reports CEP08007, CEP08027, CEP08028].
TABLE 1Recent reported LCD measurements from major CT manufacturers.ContrastSliceColorFluxIndexScannerContrastPin SizeDoseThicknessmAsCodeIndex500A0.3%4 mm10 mGy10 mm90red900400B0.3%5 mm16 mGy10 mm180blue1,4401,000C0.3%2 mm40 mGy10 mm350green3,600400D0.3%6 mm7.8 mGy 10 mm105yellow657source: NHS Purchasing and Supply Agency, Buyer's Guide, CT ScannersIt is instructive to convert these reported measurements to Contrast Index and Flux Index values and show them on an ExLCD graph (FIG. 6) based on the above definitions of ExLCD Contrast Index and Flux Index.Human Observer Method
Currently, LCD is determined by scanning the CATphan under selected protocol techniques and reconstructing the image(s). One or more human observers are then presented with the images to render an opinion regarding the smallest object they believe is visible and therefore detectable for the 0.3% contrast set. For the reported measurements described above, it is not clear whether a single observer or multiple observers were used. It is also not clear how the specific protocol was selected to derive the reported specification. At the present time, it is believed that all of the CT manufacturers except one use the observer method.
Statistical Method
At the present time, it is believed that the statistical method is used only by one CT manufacturer. The statistical method for LCD avoids the problems associated with human observers. The method relies only on noise measurements in a reconstruction. It does not use a phantom with actual contrast objects. It analyzes image noise in a specific manner that determines the amount of contrast needed to detect an object of a given diameter relative to the background with a stated level of confidence. Because the assessment is made by the computer and not a human observer, the method is highly repeatable and reproducible. However the statistical method cannot differentiate contrast performance resulting from non-linear reconstruction methods since only a noise image is evaluated. The performance of the system relative to how well the original low contrast object is preserved cannot be determined. As discussed in more detail later, this is true of any noise analysis method that does not measure an actual object.
Quantum Noise Limited
An imaging system is said to be “quantum noise limited” if, for all practical purposes, the only source of image noise is the statistics of finite x-ray quanta. In the context of Equation 13, a quantum noise limited system is one in which the electronic noise is absent, i.e., when. The plots in FIG. 3 illustrate the S/N as a function of relative x-ray Flux Index. In a log-log plot, the S/N ratio for a quantum noise limited system (green trace) will be represented by a straight line whose slope is ½. If electronic noise (system noise) is present the overall S/N will be significantly impacted only for lower flux values as shown by the red trace in FIG. 3.
It is reasonable to predict that a contrast measure over the full range of scanner protocols and body sizes will have a form similar to the S/N as shown in FIG. 3. For example, a scanner may exhibit contrast measurements such as those shown in the upper plot in FIG. 7. Then it would be possible to accurately characterize the contrast performance of the CT scanner with a curve such as the red one shown in the lower plot in FIG. 7.
With the current LCD method, however, a scanner is characterized with only ONE contrast measurement taken at a single protocol, illustrated by the bold red + and the dotted vertical line in the upper plot in FIG. 8. This single measurement does not characterize the contrast performance of the scanner. In fact, it significantly misconstrues the true contrast performance of the scanner. As shown in the lower plot in FIG. 8, the single protocol measurement implies contrast performance that follows a quantum noise limited curve defined by the single measurement as shown by the dashed line in the lower plot in FIG. 8. The inaccuracy of the single protocol contrast performance curve is illustrated in FIG. 9. Additionally, the current LCD methods do not adequately handle smaller pins, those that are impacted by system blurring, i.e. the Modulation Transfer Function (MTF). The profiles in FIG. 10 illustrate the problem with smaller pins. In FIG. 10 only pin sizes, 15, 7, 5, 3 and 2 mm are shown.
Conventional detectability methods that are based only on a noise analysis such as the statistical method, noise power spectrum, simple-pixel standard deviation or matched filter standard deviation all can over estimate the performance of a reconstruction process that alters the contrast of the test object. Given reconstruction processes that limit spatial bandwidth of both noise and object, conventional detectability methods will not account for changes in the assumed object. For example assume that a small pin in an LCD test phantom is exactly a cylinder with a 2 mm diameter and a contrast of 0.3%. If perfectly reconstructed, image pixels within the area of the pin will have an average contrast of 0.3% and all pixels outside this region will be 0%. However the MTF of the system will blur the pin especially at its edges and spread some of its contrast into pixels beyond the original geometric boundary. This results in a reduction in average contrast within the pin region.
From the narrative above, we obtain an intuitive sense about the inaccuracies of the single protocol LCD method. These inaccuracies occur for one or more of the following reasons that will be described in more detail later in this document.
1. human observer variation
2. finite pin size selections
3. selection of protocol
4. presence of system (electronic) noise
5. impact of system blurring (MTF) on smaller pins
The low contrast detectability (LCD) performance of a CT system is a critical performance characteristic, providing a measure of the scanner's ability to produce high quality images at the lowest possible x-ray dose. Because it is increasingly important to utilize lower dose protocols in present day CT scanners, it is now critical that LCD be measurable over the entire range of protocols and body sizes.
In the lower graph in FIG. 7 we illustrate an ExLCD contrast performance curve for a typical (simulated) CT scanner. CT systems vary in their contrast performance based on the following system characteristics:
1. overall dose/quantum efficiency
2. system/electronic noise
3. system blurring (MTF)
4. non-linear reconstruction methods
The dotted and dashed traces in the upper plot in FIG. 11 illustrate qualitatively how the contrast performance curve is impacted by some of these system characteristics. The lower plot illustrates the significance of those performance curve variations relative to dose. In that plot, the red line intersections show the relative dose required to achieve similar image quality on each of the respective scanners.
The three colored or shaded traces in the upper plot in FIG. 12 illustrate how three representative CT scanners might be compared. In the lower plot, the representative CT scanner performance curves are overlaid on the error region of the single protocol contrast method, illustrating that the inaccuracies of the current LCD method may effectively prohibit true differentiation of the contrast performance between CT scanners.